An electric power steering apparatus which provides a steering mechanism of a vehicle with a steering assist torque (assist force) by means of rotational torque of a motor applies driving force of the motor as the steering assist torque to a steering shaft or a rack shaft by means of a transmission mechanism such as gears or a belt through a reduction mechanism. In order to accurately generate the steering assist torque, such a conventional electric power steering (EPS) apparatus performs a feedback control of a motor current. The feedback control adjusts a voltage supplied to the motor so that a difference between a steering assist command value (a current command value) and a detected motor current value becomes small, and the adjustment of the voltage supplied to the motor is generally performed by an adjustment of a duty of a PWM (Pulse Width Modulation) control.
A general configuration of a conventional electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft) 2 connected to a steering handle (a steering wheel) 1 is connected to steered wheels 8L and 8R through reduction gears 3, universal joints 4a and 4b, a rack and pinion mechanism 5, and tie rods 6a and 6b, further via hub units 7a and 7b. Further, the column shaft 2 is provided with a torque sensor 10 for detecting a steering torque of the steering wheel 1, and a motor 20 for assisting the steering force of the steering wheel 1 is connected to the column shaft 2 through the reduction gears 3. Electric power is supplied to a control unit (ECU) 100 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal is inputted into the control unit 100 through an ignition key 11. The control unit 100 calculates a steering assist command value of an assist (steering assist) command based on the steering torque Th detected by the torque sensor 10 and a vehicle speed Vel detected by a vehicle speed sensor 12, and controls a current supplied to the motor 20 based on a current control value E obtained by performing compensation and so on with respect to the steering assist command value. Moreover, it is also possible to receive the vehicle speed Vel from a CAN (Controller Area Network) and so on.
In such the electric power steering apparatus, the control unit 100 is, for example, configured as shown in FIG. 2 disclosed in Japanese Published Unexamined Patent Application No. 2002-369565 A.
In FIG. 2, the motor 20 that generates the steering assist torque of the electric power steering apparatus is driven by a motor driving section 21, the motor driving section 21 is controlled by the control unit 100 shown by a two-dot chain line, and the the steering torque Th from the torque sensor 10 and the vehicle speed Vel from the vehicle speed detecting system are inputted into the control unit 100. A voltage Vm between the motor terminals and a motor current value i are measured and outputted in the motor 20.
The control unit 100 comprises a torque system control section 110 that performs the control by means of the steering torque Th and is shown by a broken line, and a motor system control section 120 that performs the control related to a drive of the motor 20 and is shown by a dashed line. The torque system control section 110 comprises an assist amount calculating section 111, a differential control section 112, a yaw-rate convergence control section 113, a robust stabilization compensating section 114, and a self aligning torque (SAT) estimation feedback section 115, and includes an adding section 116A and 116B and a subtracting section 116C. Further, the motor system control section 120 comprises a compensating section 121, a disturbance estimating section 122, a motor angular speed calculating section 123, a motor angular acceleration calculating section 124, and a motor characteristic compensating section 125, and includes an adding section 126A and 126B.
The steering torque Th is inputted into the assist amount calculating section 111, the differential control section 112, the yaw-rate convergence control section 113 and the SAT estimation feedback section 115, which all input the vehicle speed Vel as a parameter. The assist amount calculating section 111 calculates an assist torque amount based on the steering torque Th. The yaw-rate convergence control section 113 inputs the steering torque Th and a motor angular speed ω, and puts a brake on an action of the steering wheel whirling in order to improve the convergence of the vehicle yaw. Further, the differential control section 112 improves the control responsibility near a neutral point of the steering wheel, and achieves smooth steering. The SAT estimation feedback section 115 inputs the steering torque Th, a signal made by adding an output from the differential control section 112 to an output from the assist amount calculating section 111 in the adding section 116A, the motor angular speed ω calculated in the motor angular speed calculating section 123 and a motor angular acceleration α, estimates an SAT, performs signal processing of the estimated SAT by means of a feedback filter, and gives an appropriate road-surface information to the steering wheel as reaction force.
Further, a signal made in the adding section 116B by adding an output from the yaw-rate convergence control section 113 to a signal made in the adding section 116A by adding an output from the differential control section 112 to an output from the assist amount calculating section 111 is inputted as an assist amount AQ into the robust stabilization compensating section 114. The robust stabilization compensating section 114 is, for example, a compensating section shown in Japanese Published Unexamined Patent Application No. H8-290778 A, eliminates a resonance frequency peak of a resonator system comprising an inertial element and a spring element included by the detected torque, and compensates a phase shift of a resonance frequency to inhibit the responsibility and the stability of a control system. An assist amount Ia that is capable of transmitting the road-surface information to the steering wheel as reaction force is obtained by subtracting an output of the SAT estimation feedback section 115 from an output of the robust stabilization compensating section 114 in the subtracting section 116C.
Moreover, the motor angular speed calculating section 123 calculates the motor angular speed abased on the voltage Vm between the motor terminals and the motor current value i, and the motor angular speed ω is inputted into the motor angular acceleration calculating section 124, the yaw-rate convergence control section 113 and the SAT estimation feedback section 115. The motor angular acceleration calculating section 124 calculates the motor angular acceleration α based on the inputted motor angular speed ω, and the calculated motor angular acceleration α is inputted into the motor characteristic compensating section 125 and the SAT estimation feedback section 115. The assist amount Ia left by the subtraction of the output of the SAT estimation feedback section 115 from the output of the robust stabilization compensating section 114 is added to an output Ic of the motor characteristic compensating section 125 in the adding section 126A, and the adding signal is inputted as a current command value Ir into the compensating section 121 comprising a differential compensating section and so on. A signal made in the adding section 126B by adding an output from the disturbance estimating section 122 to a current command value Ira compensated in the compensating section 121 inputted into the motor driving section 21 and the disturbance estimating section 122. The disturbance estimating section 122 is such an apparatus as shown in Japanese Published Unexamined Patent Application No. H8-310417 A, is capable of maintaining a motor control characteristic desired on an output standard of a control system based on the signal made by adding the output from the disturbance estimating section 122 to the current command value Ira that is a control target of a motor output and is compensated in the compensating section 121 and the motor current value i, and prevents the loss of the stability of the control system.
In such the electric power steering apparatus, vehicles provided with a parking support function (parking assist) and switching an automatic steering mode and a manual steering mode recently appear. The vehicle provided with the parking support function sets a target steering angle based on data such as a camera (an image), a distance sensor and so on, and performs automatic control according to the target steering angle.
In the electric power steering apparatus having the well-known functions of the automatic steering mode (the parking support mode) and the manual steering mode, back parking and parallel parking are hitherto performed automatically by controlling an actuator (a motor) based on the relation between a vehicle moving distance and a steering angle stored beforehand.
Thus the conventional electric power steering apparatus stops the automatic steering control when a driver steers a steering wheel in the automatic steering mode and the steering torque is judged to exceed a predetermined value set beforehand. However, when performing the judgment only by comparing the output of the torque sensor with the predetermined value, there is a problem that the output of the torque sensor exceeds the predetermined value temporarily because of a noise of the torque sensor or an inertial torque of the steering wheel appearing in the case of a tire stepping on a pebble or automatic steering by the motor, and each time the automatic steering control is made stopped. When the predetermined value is set to high in order to avoid such an inconvenience, not only does the interaction between the automatic steering mode and the manual steering mode give uncomfortable feeling to the driver, but also there is a possibility that the automatic steering control is not immediately stopped even if the driver steers the steering wheel in the automatic steering control.
As an automatic steering apparatus solving such a problem, for example, Japanese Patent No. 3845188 B2 (Patent Document 1) is proposed. The apparatus disclosed in Patent Document 1 sets plural kinds of the predetermined values and changes a predetermined time corresponding to each predetermined value in an automatic steering apparatus for a vehicle that comprises a moving locus setting means for storing or calculating vehicle moving locus to a target position, an actuator (a motor) for steering a wheel, a steering torque detecting means (a torque sensor) for detecting a steering torque that a driver applies to a steering wheel, and an actuator control means for controlling the actuator drive based on the moving locus set by the moving locus setting means and at the same time stopping an actuator control based on the moving locus when the steering torque over the predetermined value set beforehand is detected for more than the predetermined time.